1. Field of the Invention
The present invention relates to an automatic gain control device which controls the playback signal level in optical memory devices which record, playback, and erase information by irradiating an optical data storage medium with a light beam.
2. Description of the Prior Art
Examples of conventional optical memory devices include a magneto-optical memory device which records information on a magneto-optical disk. The magneto-optical disk used in the aforementioned magneto-optical memory device is formed with a magnetic thin film with an axis of easy magnetization in a direction perpendicular to the film surface, enabling information to be recorded according to the magnetic orientation in minute ranges.
When recording information on magneto-optical disks of this type, the magnetic thin film is irradiated with a focused laser beam with a diameter of, for example, approximately 1 micrometer. Thus, the temperature of that part of the thin film irradiated by the laser beam rises locally, and the magnetic orientation of the film becomes more easily changed. At this point, it is possible to record and erase information by simultaneously applying an external magnetic field to change the magnetic orientation to the desired orientation.
Furthermore, when information recorded in the aforementioned manner is played back, the magnetic film is irradiated with a laser beam of a strength which will not cause the temperature to rise. Thus, the plane of polarization of the linear polarized light in the transmitted light and reflected light of the laser beam incident upon the magnetic thin film is rotated by the Faraday effect and the Kerr effect. Therefore, by wave-detecting this transmitted light and reflected light using an analyzer, and converting the light to electrical signals, a pulse-type playback signal is obtained.
Normally, the signal level of this playback signal has a tendency to fluctuate due to variations in the playback conditions. Therefore, in a conventional optical memory device, an automatic gain control device is provided so as to control the level of the playback signal at a constant level.
In FIG. 4, a conventional automatic gain control device 10 in the aforementioned optical memory device is shown, which includes a gain control amplifier 11 in which the amplification varies based on the amplification control signal ACS. Also provided are a signal level detection circuit 12 which outputs a voltage according to the level of the signal outputted by gain control amplifier 11, and an operational amplifier 13 which compares the difference between the output signal level of gain control amplifier 11 detected by this signal level detection circuit 12 and a predetermined reference signal level from a reference signal generator 13'. Operational amplifier 13 outputs the amplification control signal ACS, which is fedback to gain control amplifier 11 thereby maintaining the playback signal at a constant level.
Data is not necessarily stored across the entire range to which recording is possible on the optical data storage medium, and it is possible that there are wide areas to which data is not recorded.
Thus, when the playback position is at a place on the magneto-optical disk where data is not recorded, the automatic gain control device 10 tries to respond to a LOW playback signal containing no pulse signal such that the amplification of gain control amplifier 11 becomes excessively great. In this case, when the playback position enters an area to which data is recorded on the magneto-optical disk, amplification is not able to immediately track the playback signal level when the playback signal containing the pulse signals is applied to gain control amplifier 11.
Moreover, as shown in FIG. 5, immediately after the playback signal S1 containing the pulse signal is inputted, the output signal S2 of gain control amplifier 11 is significantly increased. In addition, a normal playback signal cannot be obtained for a certain period of time until the amplification tracks the playback signal level and attains a suitable amplification level.
Furthermore, if scratches or foreign matter are present on the surface of the optical data storage medium, a defect pulse 14 as shown in FIG. 6 may be contained in the playback signal S1'. When automatic gain control device 10 responds to this defect pulse 14, the amplification of gain control amplifier 11 becomes very low. A significantly longer period is required for this low amplification to return to normal amplification than in the aforementioned case of excessive amplification.
As understood from the above, after the defect pulse 14' is generated, the output signal S2' of gain control amplifier 11 drops significantly, and it is, again, not possible to obtain a normal playback signal.
As thus described, in the prior art automatic gain control device 10 for an optical memory device, the amplification and playback signal level of gain control amplifier 11 have a tendency to vary significantly due to the influences of unrecorded data areas, scratches, or foreign material on the optical data storage medium, thereby creating errors in the playback data, and thus reducing reliability.
In order to solve the aforementioned problem, it is conceivable to, for example, prerecord a dummy signal at the beginning of the area to which data is recorded so as to provide a time allowance sufficient for the change in the amplification of automatic gain control device 10 to respond. But this to creates wasted recording area and invites an effective reduction in recording density and a reduction in recording and playback speeds.